1. Technical Field
This application relates generally to formation fluid analysis as may be accomplished downhole or at a surface. More particularly, this application relates to detection and identification of classes of interest within a formation fluid using a chromatography-based device.
2. Background Information
Downhole fluid analysis (DFA) is a rapidly growing discipline in wireline logging and has become a keystone in reservoir evaluation. DFA addresses the failed and overly optimistic assumption that oil reservoirs contain of “one giant tank of homogeneous hydrocarbon.” Beneficially, DFA can be used to find compositional gradients as well as to identify compartments. Such analyses are typically based on bulk optical spectroscopy of samples of formation fluid to determine concentrations or ratios of components in sample fluid.
More recently, sophisticated optical measurement techniques have been developed to, among other things, determine methane (C1), ethane-propane-butane-pentane (C2-C5), and heavier hydrocarbon molecule (C6+) compositions for hydrocarbons and gases. Current downhole analysis techniques, however, do not provide quantitative measurement of the individual hydrocarbon moieties for C2, C3, C4, C5 and molecules with more than six carbon atoms are indistinguishable.
Because different materials have different absorption characteristics, it becomes possible to make a determination as to what materials comprise the fluid sample, provided that the spectra of the materials which might be in the fluid sample are known. To that end, the spectra of water, gas, and several different oils are found in accordance to techniques generally well known in the art.
Using the absorption spectra of water, gas, crude and refined oils, and drilling fluids (lights), a least-squares analysis can be used to determine the components of the fluid sample. Alternatively or in addition a principal component analysis can also be used in a similar manner to determine the components of the fluid sample.
One such process referred to as “de-lumping” operates to determine compositional data from optical absorption spectra of samples of formation fluid in order to estimate molar distribution of the components in the component groups. Weight fractions for such components are then derived from molecular weights and the derived mole fractions.
For example, black oil de-lumping can be accomplished based on composition versus saturation pressure tables. Alternatively, de-lumping can be accomplished using tables of liquid and vapor compositions versus the liquid phase's gas/oil ratio (Rs) and/or the vapor phase gas/oil ratio (Rv) for the de-lumping process for greater accuracy. Unfortunately, processing the data in this manner requires substantial processing time. Consequently de-lumping precludes any compositional evaluations in real-time or even near real-time.
There are several approaches possible to make collected data quantitative and provide clients with information about the amount (weight or mole percentage) of the components of interests in the analyzable mixture:
In an absolute calibration method, a calibration plot is obtained in which the detector response versus the amount of the injected component could be utilized for quantification of the chromatogram or spectrogram. However, this method requires that conditions of the analysis during the calibration procedure and during the experiments are identical, which is a challenge for the variety of downhole conditions worldwide.
An internal standard method requires that some amount of the “standard” is carried together with the main module. During the analysis, the standard is mixed together with the formation fluid and injected into the separation module (e.g., chromatograph). This method requires bringing the “standard” downhole, mixing it with the sample of the formation mixture, identification of the “standard” response and its quantification, which is often very challenging.
In a case when it is difficult to resolve the added internal standard from other eluted peaks, a controlled admixture can be added to the second consequent chromatogram. Knowing the detector response factor, it is possible to quantify the amount of the analyzable component. Although this method is free of some of the problems of the internal standard method, it still requires bringing downhole the admixture that will be added.
A method of internal normalization assumes that all components elute from the column and are detected. The sum of the areas of all peaks represents 100% of the total concentration and the amount of the component of interest can be estimated from the proportion. However, in case of analysis of such a complex mixture as a crude oil, not all components will elute from the column (e.g., resins, asphaltenes, and very heavy saturates). Furthermore, integration and summation of all eluted peaks introduces significant error in the analysis results.